Posted: 6/4/2015 6:25:19 PM EDT
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I just built a 2m J Pole and I cannot get the SWR's below 3:1. Any suggestions?
for 147mhz Long arm - 57.528" Short arm - 19.094" Spacing - 1.795" Feed point - 1.877" Stub below J - 24" |
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Quoted:
If the center is at 130, the main stub sounds tall That's what was coming to mind. I'll chop off little by little and see what happens.mi got enough pipe to build another if I want Making it for 147mhz cuz that the repeater I use this radio with 99.999% of the time. 147.060 so Imam trying to get it good on 147.5-147.6. The radio will rarely be moved from that freq |
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For reference, my J-pole has the long arm at 60-3/8", the short arm at 21", the spacing at 2-1/4", and the coax connection at 3-7/8".
The material is 1/2" Schedule 40 hard copper tubing with sweat fittings and all dimensions are measured centerline-to-centerline. The SWR is 1:1.2 @ 144 MHz and 1:1.3 @ 148 MHz. It took a lot of fiddling to get it there, but I wanted to see just how flat I could get it. I kind of cheated by using end caps on the two arms and worked them up and down until the match was achieved. The coax was held in place by copper pipe straps. Everything was finally soldered together after I was satisfied. |
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Quoted:
1/4 wave ground plane is a whole lot easier.  Yes - and has a bit higher gain 99% of the time as well as a cleaner radiation pattern!
(Curse The Devil that unleashed antenna designs with common mode coax currents upon the poor unsuspecting ham community!!!) |
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Quoted:
1/4 wave ground plane is a whole lot easier. You say this so often I went and did a little research and interestingly, it appears to be pretty simple to tune these for 75 Ohms. I use 75 Ohm hardline cable to my 2m antenna, so I will definitely be making a 1/4 wave and checking it's performance vs the j-pole. |
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Another DIY base antenna with some potential for gain is the W7LPN vertical collinear. I built a 15' version a while back, about $25 for parts including the ferrites for the feedline choke. You can make them from various alternative materials copper tube, hardware cloth, pvc & aluminum foil etc Can make for a fun project..
Nick |